In mineralized tissue research it is often useful to carry out experiments on in vitro tissue cultures. For example, various compounds can be added to investigate their effect on bone formation. It is extremely important to characterize the in vitro tissue and to compare its composition with that of normal in vivo tissues. Electron energy loss spectroscopy is being used to find the relative distributions of phosphorus and calcium in cultured mouse condyle. Preliminary results indicate that the composition of certain newly formed mineralization samples is different from that of hydroxyapatite.